Total and Bioavailable Pb Content in Urban Paddy Rice Soils in Niger State , Nigeria

The total and plant available Pb content of forty composite soil samples from eight cultivated rice paddy soils from Niger State were investigated using Atomic Absorption Spectrophotometric technique, after their digestion with aqua regia and extraction with EDTA respectively. Varying Pb contents were obtained; the overall total Pb mean value was 12.6  13.1 mg/kg, while the plant available Pb had mean concentration of 5.0  3.1 mg/kg. Although the studied soil samples in this research were not polluted, they indicated a considerable Pb contamination that could serve as baseline data for lead level in paddy soils in Niger State. Hence, the need for constant monitoring of paddy soils to safe guide the possible risks that could arise due to bio accumulation of Pb above the safety level and its consequence transfer through the food chain.


Introduction
Urban soils are often contaminated (Mielke, 1994) with Pb, due to its ubiquitous nature in the environment (Nriagu, 1998).In urban areas, Pb contamination is generally higher in high traffic zones, concentrated industrial areas and older housing stock with lead containing paint (USEPA, 1998).Since Pb does not biodegrade or decay, concern for its significant role as a primary contributor in soil contamination in urban areas still persist, despite the fact that the addition of lead to gasoline and paint has been phased out in the 1970s ( Clark et al., 2006).In urban gardens, fruits and vegetables grown in contaminated soils may also become contaminated as a result of plant uptake of Pb from soils or direct deposition of leaded dust onto plant surfaces (Rahlenbeck et al., 1999).
Pb is a toxic heavy metal whose extensive use has given rise to environmental contamination and implicative health challenges in many parts of the world.WHO (2009) documented that Pb exposure accounts for about 0.6% of the global burden of diseases, particularly in developing nations.ATSDR (2008) also classified Pb as the second most dangerous metal on the priority list of the U. S. Environmental Protection Agency.Acute exposures to Pb may cause gastrointestinal disturbances, hepatic and renal damage, hypertension and neurological effects that may consequently result to convulsions and death (IPCS, 1995).
Although, Laidlaw and Fillippelli (2008) reported that sampling strategies encouraged surface soil collection, when analyzing for soil Pb because of Pb accumulation in insoluble forms on the topsoil.However, proper understanding of the risks of soil Pb is particularly difficult in gardens used for growing food plants, because of the multiple ways by which soil Pb is ingested, inconsistent recommendations from various extension agencies, the fact that Pb testing services are not easily accessible and due to lack of standard EPA guidelines for Pb in garden soils used for growing food plants (Witzling et al., 2011).
Currently the use of acidic or chelating agents to dissolve trace metals from solids to solution has been on focus.Since most pollutant inputs in urban top soils are not silicate bound (Nomeda et al., 2004), a pseudo-total digestion methods of strong acids, that do not involve dissolution of silicates by hydrofluoric acid are used (Novak et al., 2003;Farmer et al., 1997).This type of analysis using aqua regia is considered adequate (Hseu et al., 2002;Chen and Ma, 2001;Loncaric et al., 2010), and is found useful in the estimation of the maximum element availability to plants (Vercoutere et al., 1995).Moreover, among the chelating agents used for heavy metal extraction from polluted soils, EDTA has been considered to be the most effective in solubilising soil -www.ccsenbound Pb (Samani e (2006) obs soils.
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Locatio
Niger State land mass central zon and Suleja

Soil Sa
The soil sa 1).Sample  ixture was tigh

Conclusion
Though still at very low levels of insignificant concern, Pb was found to be present in all the rice paddy soils of this study.However, Iyaka and Kakulu (2009;2012) had reported no detectable Pb contents in background level of all their control soil samples in the study of urban agricultural soils in Niger State, Nigeria.Thus, the obtained results from this study signify accumulation of Pb above the background level for paddy soils locations of this research in Niger State.Furthermore, high concentrations of Cd, Pb and Hg have been identified with reduction in soil fertility and agricultural output (Lokhande and Kalkar, 1999).Hence, the need for constant monitoring of agricultural/paddy soils to safe guide the possible risks that could arise due to bio -accumulation of Pb above the safety level and its consequence transfer through the food chain.

Figure
Figure ampling and P amples were co es were collect re-cleaned wit ari and Chandr h point, three s oint.The thre of the samplin nd twenty sub ual removal of disaggregated steel sieve and reated sample d in well-labele tical Methods of aqua regia w